Engine lathes are employed for machining operations wherein a workpiece is supported and turned on the lathe generally on a horizontal axis while a cutting or abrading tool is advanced into cutting engagement with the surface of the turning workpiece to form a circumferential profile on the workpiece.
A cutting tool for a lathe usually has a shank portion that is clamped in a tool holder mounted on the lathe carriage lateral to the workpiece. The shank of the clamped tool is usually oriented in the horizontal plane passing through the rotational axis of the workpiece. A cutting edge is located on one end (point) of the tool and is usually ground to have a particular shape for the job at hand. When the tool is clamped in the tool holder, the cutting edge is oriented toward the surface of the workpiece to be cut using the tool (referred to herein as the target surface).
The tool holder can be moved via a cross-feed mechanism toward or away from the workpiece and along a "longitudinal move" substantially parallel to the rotational axis of the workpiece. With many engine lathes, such movements of the cutting tool are motorized to permit, for example, precise cutting of threads in the surface of the workpiece.
Certain industrial operations such as rolling and roll-extruding require machinery having rollers. Generally, such rollers have a circular cross-section, such as cylindrical rollers, which can be made using an engine lathe. For example, plastic or elastomeric sheeting can be extruded between opposed counter-rotating rolls of a roller apparatus. In some situations, it is desirable to form such sheeting with integral ribbed patterns on one or both planar surfaces, which requires that at least one of the rollers in the roller apparatus have a surface grooved in a complementary pattern. To form sheets having straight ribs running the length of the sheeting, complementary circumferential grooves are required in the corresponding roller, which are easily cut into the roller surface using a conventional engine lathe. However, if the sheet design requires serpentine or wavy ribs, the required complementary serpentine or wavy grooves in the roller surface are virtually impossible to cut using a conventional engine lathe capable only of unidirectional movement of the cutting tool.
At least one other type of apparatus has been used for cutting such grooves, but with limited success. In particular, the workpiece is mounted on a multi-axis CNC milling machine employing a rotating cutting tool for cutting the grooves. One problem with using this approach is the extremely complex mechanism required to manipulate the workpiece in the required manner relative to the rotating cutting tool. Such complex mechanisms have inherent cumulative inaccuracies that can prevent the attainment of desired close tolerances in the finished product. The main problem with this approach is that the milling tool must have a diameter less than the width of the desired groove. For example, if a groove has a specified width of 1 mm, the milling tool diameter must be less than 1 mm. Such narrow milling tools necessarily have extremely small cutting teeth and are relatively flexible, resulting in a high rate of tool wear and breakage, poor finish of the groove in the workpiece, and poor adherence to specified tolerances. If the workpiece is fabricated from such hard materials as steel or stainless steel, cutting narrow serpentine-patterned grooves therein using such an apparatus is essentially impossible. In other words, a lathe operation is the best known method of cutting circumferential serpentine grooves in the surface of a workpiece.
When cutting grooves of a repeating serpentine pattern in the surface of a workpiece using an engine lathe, it is necessary that the cutting tool undergo several types of simultaneous periodic movements as it is urged into cutting engagement with the surface of the workpiece turning about its rotational axis. First, the tool must be moved in a reciprocating (limited periodic linear back and forth) manner along a line or path generally parallel to the target surface of the workpiece while keeping the longitudinal (or shank) axis of the tool oriented substantially normal to the target surface. The period of reciprocation must be coordinated with the angular velocity of the turning workpiece so as to form a continuous groove with the desired integral number of serpentine waves around the circumference of the workpiece. Since more than one pass of the tool over the circumferential target surface of the workpiece is virtually always required to cut any desired profile, the period of reciprocation of the tool must be an integral multiple of the circumference of the workpiece to form an integral number of complete wave cycles around the circumference of the workpiece.
Second, the cutting tool must be moved in an oscillatory manner (limited periodic clockwise/counterclockwise rotational motion about the longitudinal or shank axis of the tool) in synchrony with (having the same period as) the reciprocating movement of the cutting tool. The oscillatory motion is required because proper performance of the cutting tool requires that the cutting edge be urged against the material being cut at a substantially constant angle, generally in a normal orientation. If the cutting tool underwent only a reciprocating motion while cutting a serpentine groove circumferentially around the workpiece, the angle of the cutting edge of the tool relative to the serpentine pattern would always be changing, resulting in poor cutting performance and probable fracture of the cutting tool. Hence, the cutting tool must be rotated periodically on its shank axis in both directions (i.e., oscillated) in synchrony with the reciprocating movement of the cutting tool during the cutting operation in order to ensure that the cutting angle remains substantially constant at all instantaneous positions of the cutting edge on the serpentine pattern.
One profiling machine capable of imparting combined reciprocating and oscillatory movement of the cutting tool used on an engine lathe is known to have been patented. Geer (U.S. Pat. No. 2,010,662) discloses an apparatus in which a cutting tool is mounted in a tool holder capable of limited rotational (oscillatory) rotation about the shank axis of the tool. The tool holder, in turn, is mounted to a block allowing the tool to be reciprocated in the horizontal plane along a line parallel to the rotational axis of the workpiece held in the engine lathe. In addition, the tool holder can be reciprocated in a direction transverse to the rotational axis of the workpiece. The combined movements of the cutting tool are dictated by cams rotated by a complex gear and driveshaft mechanism. The period of oscillation in the Geer apparatus is determined by assembling on the lathe gears having the appropriate gear ratio, which requires a complicated series of operational steps.
A key shortcoming of the Geer apparatus is its reliance upon a complex combination of gear trains, drive shafts, couplings, and cams to effect reciprocation and oscillation of the cutting tool. Because of the resulting large number of dynamically interacting parts, the resulting backlash, stacking of tolerances, and multiple clearances would render the apparatus incapable of achieving close machining tolerances. For example, a large roller having repeating courses of parallel serpentine grooves used to roll-extrude plastic sheeting used in manufacturing separator envelopes for plates used in storage batteries requires a groove depth-to-width ratio of 4:1 or greater, where the width of the groove is approximately 0.25 mm. The cutting tool for such a machining operation is necessarily very delicate and fragile. The positioning inaccuracies imparted to the combined reciprocating and oscillatory motions of the tool by an apparatus according to Geer would cause the cutting tool to rapidly fracture when used to cut such a groove.
A second disadvantage with the Geer apparatus is the difficulty inherent in using such a complex apparatus as an "add-on" accessory to an existing conventional engine lathe. A number of special modifications to the lathe would be required to fit the Geer device to it. Also, the Geer device comprises a large number of extraneous parts attached to the lathe that in combination lack the structural rigidity of the lathe itself. Modern engine lathes are constructed of heavy rigid components to minimize flex and dimensional distortion. As a result, it would be advantageous to utilize as much of the structural framework of the lathe itself whenever a special machining apparatus is retrofitted to the lathe.
Hence, there is a need for a method and apparatus for cutting close-tolerance serpentine-shaped grooves in a circumferential target surface of a workpiece supported by and turned on an engine lathe.
There is also a need for a method and apparatus for cutting deep, narrow serpentine-shaped grooves in a circumferential target surface of a workpiece supported by and turned on an engine lathe.
There is also a need for such an apparatus that can be retrofitted easily to an existing engine lathe without any significant deterioration of dimensional tolerances otherwise achievable with the lathe alone.
There is also a need for such an apparatus that uses few additional parts that must be fitted to an existing engine lathe.